Through the years, seeds have been coated for a variety of reasons: to provide a uniform seed size so as to facilitate planting (see U.S. Pat. No. 3,905,152), to aid in germination, to inhibit herbicidal damage, to delay germination, to control pests (see U.S. Pat. No. 5,849,320), to retard erosion (see U.S. Pat. No. 4,192,095), to promote water accumulation and absorption and to provide immediate fertilization.
Some coatings are applied merely by dusting the seeds with a particulate solid, which has sometimes been used, for example, to apply a fungicide to seeds. Other coatings are applied using a water-soluble or water dispersible adhesive. Water-based adhesives have often been used because of concerns that a non-aqueous system may damage the seed. Water-based adhesives also have been used to minimize any adverse impact the coating may have on germination. Such adhesives often adhere a particulate material to the seed for any of the above-noted purposes. For example, by including nutrients in the coating, the immediate soil environment of the seed can be favorably changed to promote germination and/or growth of the plant.
One advantage of using an adhesive binder, such as a polymeric material, in the seed coating is that loss of coating material is generally minimized and surface dusting is kept to a minimum.
Porter, F. E., Chemtech, May 1978: 284–287 describes using a two-part polyurethane polymer (polyurea varnish/acetone solvent) to coat legume (alfalfa) seeds with lime. See also U.S. Pat. No. 3,808,740.
Ros, C. et al., Seed Sci. & Technolo., 28: 391–401 describes efforts to coat rice with phosphorus fertilizers using a methyl cellulose glue.
U.S. Pat. No. 4,251,952 describes a seed coating made from an aqueous mixture of sugar and a dispersed preformed water-insoluble polymer, cured simply upon the removal of water, i.e. by drying. Particularly suitable among such preformed polymeric materials are vinyl acetate polymers, especially a vinyl acetate polymer that has been hydrolyzed to form a polyvinyl alcohol. Such polymers are represented by a number of commercially available white glue formulations. The patent also describes adhering an agriculturally acceptable particulate material to the coating simply by mixing the seed with the material after coating but prior to curing (drying).
U.S. Pat. No. 4,735,015 to Schmolka et al., describes enveloping seeds in a protective coating (0.5 to 3.0 mm thick) of a film-forming polyoxyethylene-polyoxybutylene block copolymer. The coating may be applied by depositing a melt of the copolymer on the seed, or by dissolving the copolymer in a solvent, or dispersing it in another liquid and spraying onto the seed. The disclosure suggests that other materials such as inert fillers, moldicides, fungicides and various nutrients such as nitrogen, potassium and phosphorous and salts thereof may also be coated onto the seeds. Such materials can be applied before, after, or with the copolymer. The patent describes alternative methods for coating the seed. Example 12 specifically describes providing a first coating of the copolymer on the seeds followed by a second coating of a 5-10-5 N—P—K fertilizer from an aqueous suspension.
Nitrogen fertilizer is often applied as a formulated (N—P—K) solid, granular or powder, or sometimes as a liquid to an area to be fertilized. The solid form is generally fast release, but it can be made slow release by various coatings. Alternatively, a reduction in nitrogen availability also can be obtained by using enzyme inhibitors. The liquid form may include both fast-acting and slow release forms of nitrogen. Some of the recognized disadvantages of such nitrogen fertilization involve run-off into rivers and streams, ammonia emissions and nitrate leaching.
The seed coat prior art, such as U.S. Pat. No. 4,735,015, however, only describes the use of fast release nitrogen in seed coatings. Unfortunately, only a very small amount of nitrogen can be used in such coatings without burning the seed, or the germinating plant. The prior art does not describe how to introduce a source of slow release nitrogen using a seed coating, as a way to enhance plant development and growth and minimize the problems of run-off and ammonia emission. The present invention proposes to supply a particulate slow-release nitrogen fertilizer as a seed coating.